The present invention relates to a control equipment for a high voltage direct current (dc) transmission system. There has been developed application of switching devices with a self-cummutaing (gate-turn-off) function to equipment of large-capacity power converters such as a high voltage dc transmission system (HVDC), static var compensator (SVC), and CVCF (power source of constant voltage constant frequency). Among these attempts, a control system for an equipment of self-commutated converter, which is one application form of such switching devices to the high voltage dc transmission system, has been studied.
Equipment of self-commutated converter generally operates as voltage sources. In the event the line voltage is abruptly changed upon the occurrence of an ac system fault, therefore, if the converter voltage is not changed correspondingly, an accident may happen because a current flows depending on the difference between the ac system voltage and the output voltage of the converter equipment. Thus, the current may exceed an allowable value of the converter equipment and the converter equipment may be damaged in some cases. To obtain a current control system with quick response as a means for preventing such an accident, a control system for making independent vector control per phase of a two-phase current resulted from transform of a three-phase alternating current (ac) has been proposed Y. Tokiwa, et al, "Application of a digital instantaneous current control for static induction thyristor converters in the utility line", PCIM '88 Proceedings.
A high voltage dc transmission system comprises a rectifier for converting an ac power to a dc power and an inverter for converting a dc power to an ac power. Although these rectifier and inverter are required to be operated in cooperation to stably and efficiently operate the high voltage dc transmission system comprising equipment of self-commutated converters, no considerations have been paid to this point in the above conventional control system.